The present invention relates to rolling mill equipment. In particular, it relates to control devices to maintain a traveling sheet or web of material, such as metal, centered upon rollers along which it is processed.
Any continuous moving web of material, as it progresses over a series of rolls, tend to move from the center of travel to one or the other edge of the rolls. This side-to-side weaving is induced by several factors. For instance, metallic strips can be rolled with a "chambered" shape, a condition in which one edge of the strip is longer than the other; materials may exhibit different degrees of elasticity from one edge to the other; segments may vary across a web; or the joint, or weld, where one continuous coil is joined to another, may be misaligned. Additional contributing factors include inadequate tension, misalignment of machine elements, excessive roll wear or taper, and inadequate structural rigidity.
Strip off tracking (this would encompass such materials as steel, aluminum, rubber, paper, textiles, etc.) forces slower line speeds, damage to equipment and material being processed and major delays when the material tracks off the roll faces completely. A common present-day solution to this problem is the use of steering rolls within the process line. Such present-day rolls are commonly designed to pivot for redirection of off tracking strips. These rolls put pressure on an edge of the moving strip to induce movement to the "tight" side. To be effective such rolls require 180.degree. of strip wrap. Experience has shown such control to be neither positive nor quick enough in response to effect total or accurate strip steering. Additionally, such steering of a strip may result in distortion, deformation, and/or stretching of the material.
These and other problems introduced by pivoted steering rolls have led to attempts to achieve web steering by the imposing forces entirely normal to the desired direction of web travel. In this regard, Avery, et al, U.S. Pat. No. 3,786,076 discloses a complex, sensor-activated steering roll which features a plurality of slats 32, each of which may be made to reciprocate throughout the rotation of a roll body 10. Such reciprocation is coordinated with the positioning of the web upon the roll body 10 to impart the desired pure axial force while the web contacts the slat-surrounded roll body 10. However, due to the fact that each slat 32 has the same null point of reciprocation within a rotation cycle of the roll body 10 and that a plurality of the slats 32, each positioned at a different stage of the rotation cycle of roll body 10 (and, hence, each having a different degree and possibly a different direction of displacement from null orientation) may simultaneously engage the traveling web, this steering roll may introduce web fiber shear stresses.